Glass manufacturing apparatus are known to produce glass sheets, for example, by a fusion down draw process. U.S. Patent Application Publication No. 2009/0107182 that published on Apr. 30, 2009 to Anderson et al. discloses an example glass manufacturing apparatus with a lower pull roll apparatus having a master motor to rotate a lower pair of rolls at a constant angular velocity. The glass manufacturing apparatus further includes an upper pull roll apparatus with upper slave motors configured to rotate an upper pair of rolls at torques that match a predetermined percentage of the measured torque of the master motor of the lower pair of rolls.
The master/slave configuration of the lower/upper pull roll apparatus of the Anderson et al. publication can be beneficial under various process applications. However, perturbations from the glass ribbon growth and sheet formation may propagate to the upper pair of rolls. For example, FIG. 1 illustrates an example graph of a master/slave configuration where the vertical axis is force and the horizontal axis is time. One plot 101 represents the force being applied to the glass ribbon by the lower rolls while the other plot 103 represents the force being applied to the glass ribbon by the upper rolls. As shown, each plot 101, 103 includes a saw-tooth force pattern with a first force pattern 105 representing glass ribbon growth and a second force pattern 107 representing separating of a glass sheet from the glass ribbon.
Moreover, known pull roll apparatus typically include pairs of pull roll members, wherein each pull roll member includes a refractory roll coverings that engage respective first and second edge portions of the glass ribbon to draw the glass ribbon from a root of a forming device. Over time, the refractory roll covering may begin to wear away, thereby changing the actual outer diameter of the refractory roll coverings.